There are many reasons why it is desirable to measure the strength of a magnetic flux or a magnetic field with high sensitivity. At present, the most sensitive magnetometers are those based on superconducting quantum interference devices (SQUIDs). These are responsive to very small changes in magnetic flux, and so can be used to measure any physical quantity that can be converted into a magnetic flux, e.g. electric currents and mechanical displacements.
SQUIDs operate on the principle of the Josephson Effect, whereby a supercurrent is able to flow through a thin insulating barrier or a weak-link between two superconductors, without a voltage drop across the barrier, as a result of quantum tunnelling of superconducting Cooper pairs. Such a junction between the two superconductors is known as a Josephson junction. There are two types of SQUIDs, DC SQUIDs and RF SQUIDS.
DC SQUIDs are formed of a loop of superconducting material, with two Josephson junctions connected in parallel on the loop. Therefore, two paths along which the supercurrent can flow are provided, each containing one Josephson junction. When a current I flows through the loop, it splits equally between the two branches of the loop. When an external magnetic field B, with an associated external flux ϕ is applied to the loop, a screening current Is begins to flow in the loop, in order to cancel the effects of the applied magnetic field. This screening current will flow in opposite directions in each branch of the loop, so the current in one branch is equal to (I/2+Is) and in the other, it is equal to (I/2−Is).
It is known that the magnetic flux which passes through a superconducting loop must be an integer multiple of the flux quantum ϕ0. Therefore, the screening current changes direction depending on whether it is more energetically favourable to cancel or enhance the magnetic flux to be a multiple of ϕ0. Therefore, the screening current varies periodically with a period of ϕ0. When the current in either branch exceeds the critical current of the Josephson junction IC, there is a voltage drop across the SQUID, which can then be measured in order to obtain a value for the flux ϕ.
Since the flux quantization in the loop is not perfect in practice, a parameter βL can be defined, given by βL=2πICLSQ|ϕ0 where LSQ is the self-inductance of the SQUID itself. The SQUID sensitivity is optimised when βL=1.
DC SQUIDs are relative magnetometers, meaning that they can only measure the change in the magnetic flux, from the change in the voltage across the SQUID. Therefore, they require some kind of calibration in order to obtain an absolute value of the incident magnetic flux.
SQUIDs have traditionally been made of metals such as niobium. However, these must be cooled using liquid helium in order to fall below the critical temperature at which they become superconducting. More recently, however, high temperature superconductors have been developed, which can operate at much higher temperatures. These are advantageous because they can be cooled using much cheaper liquid nitrogen or inexpensive cryogen-free cryocoolers.
In order to obtain improved results, it is known to use an array of SQUIDs, rather than a single SQUID. A superconducting interference filter (SQIF) is a type of magnetometer which operates in this way. Then, the combined responses of the SQUIDs can be used to provide a readout. U.S. Pat. No. 8,179,133 describes a magnetic field detector including an irregular array of SQUIDs, configured so that their combined output sums to produce a linear output (using a Fourier series), which is then used to measure a change in magnetic field strength. U.S. Pat. No. 7,369,093 discloses a similar device, where the array is configured so that the ϕ0-periodic component of the output signal is suppressed.